1. Field of the Invention
The present invention relates generally to television tuning systems and more particularly to a tuning system employing a plurality of logic gates interconnected to provide counting and decoding of a local oscillator frequency for comparison with encoded viewer input channel information.
2. Description of the Prior Art
The FCC rulings requiring equalized tuning capability for television signals in both VHF and UHF frequency bands and the increasing use of varactor tuners have provided the impetus for developing new electronic tuning systems. These new tuning systems have several advantages over the widely used mechanical turret type tuning systems, among the more important being increased operational speed, reliability and quietness. Most electronic tuning systems are of the limited channel type and are programmable to receive, with equal ease of tuning, any of a number of channels, usually about 14. A problem which is indigenous to all-channel television tuning systems is that the FCC television channel frequencies are allocated in four disconnected frequency bands, whereas the corresponding channel identification numbers (exclusively used by viewers for identification) run serially without regard to the frequency band interruptions. Thus, there are problems in translating input information in channel number form since the television reception circuitry operates in a frequency domain.
There are two types of all-channel tuning systems employing varactor tuners: a direct type which applies a selected voltage to tune the tuner independently of a received signal; and an indirect type which uses error signal information to drive the tuner until proper tuning is achieved. The former system, while less complicated, requires exacting calibration for the characteristics of individual varactors whereas the latter system may be designed around average varactor characteristics.
A paper by Doyle and Mills in BTR, Volume 15, Number 2, July, 1969, discusses some possible electronically controlled varactor tuning systems. A channel selection and indication system is described in block diagram form. It employs a varactor tuner and means for detecting the received television signal and comparing its frequency with the channel number input information. The detection methods described are a frequency synthesizer and a marker (or birdy) synchronizing synthesizer. With either detection method, a comparator compares the incoming signal frequency with the input channel number information and, at equality, operates to send a stop signal to a tuner ramp voltage drive. A phase-lock loop may be used to maintain equality between the detected signal frequency and the input channel frequency.
A paper by Dexter appearing in the Jan. 18, 1970, issue of ELECTRONIC DESIGN describes a method for improving the frequency stability of a radio tuner with a closed loop digital automatic frequency control (AFC). An electronic counter measures the local oscillator frequency, by counting with respect to a known time base, and a comparator compares the last digit in the counter with a preselected number. At equality, a signal is produced to terminate adjustment of the local oscillator frequency. Thus, fine tuning is related to the allocated frequency of the radio station and achieved without reference to the incoming signal. The system is described as an adjunct to a conventional method of coarse tuning the oscillator with the digital AFC analyzing the last digit of the derived local oscillator frequency with respect to a preselected digit. The author alludes to the broader use of digital AFC as a tuning mechansim, but doesn't go into any detail. Further, as will be apparent later, comparison of the last digit has a number of shortcomings.
A later paper by Doyle et al. in BTR, Volume 18, Number 4, November, 1972, shows an all-channel tuning system of the birdy frequency synthesizer type which includes an additional reference division to provide finer control of tuning when the tuner approaches the desired frequency. The oscillator is swept through a band of frequencies containing the desired frequency and a birdy is generated as each 6 MHz harmonic is passed. This signal is filtered and fed to a sweep control circuit. When the birdy immediately preceding the desired channel is reached, as determined by counting, an additional divider is activated to produce birdies which are 1 MHz apart.
The tuning system described in U.S. Pat. No. 3,654,557, issued Apr. 7, 1972, uses an arbitrarily selected binary encoding of channel numbers to designate a limited number of individual potentiometers for supplying corresponding preset voltages to a varactor tuner. The system input is from a keyboard which has one switch for each of the allowed programmed channel positions. Each channel position has a corresponding binary encoded number which determines the location of a corresponding preset potentiometer for coupling to the varactor tuner. This system includes a viewer controlled switch which connects clock pulses to a series of gated flip-flops interconnected to form a counter capable of indexing through the allotted group of binary numbers. A viewer, by use of this additional switch, may initiate channel indexing and, while observing the channel number display, stop the tuner at the desired channel by releasing the switch.
A paper by Sakamoto and Ichinohe in BTR, Volume 18, Number 3, August, 1972, discusses an improvement of the system in U.S. Pat. No. 3,654,557 including use of an eight bit binary encoded signal which corresponds to an input of a two digit channel number. The encoded signal is stored in a shift register and, through a series of logic gates, activates the potentiometer corresponding to the selected channel number. One signal lead from the shift register is used to determine the required tuning band and activate the appropriate tuner and another signal lead supplies a decimal decoder for driving a visual channel number display. As before, the system requires a separate potentiometer for each channel to be selected. The improvement lies in using a two digit channel number to determine the location of a corresponding potentiometer for subsequent coupling to the varactor tuner.
U.S. Pat. No. 3,509,484, issued Apr. 28, 1970, describes a digital logic system, including a prescaler gated with a divided-down clock signal, for displaying the frequency of an oscillator. The gated output is sequentially divided in a series of decade counters and the resultant frequency count is displayed. While not constituting a tuning system, the disclosure shows a digital counting method of deriving and displaying the frequency of an oscillator.
The above-mentioned related application, Ser. No. 326,724, filed Jan. 26, 1973, by Merrell and Tanaka, discloses a method and apparatus for sampling the local oscillator frequency with a counting system to derive the television channel number to which the tuner is tuned. The output of a logic gate, in which a reference clock signal gates a local oscillator signal, is used to drive a modular scaler, a units counter and a tens counter. All are presettable with the modular scaler operating in conjunction with the tens and units counters to decode the local oscillator output signal into the corresponding channel number.
Since the television spectrum is divided into four discontinuous frequency bands, labelled Low band VHF, Mid band VHF, High band VHF and UHF, different presets are supplied for each band. The band presets are sequentially changed whenever an "overcount" or "undercount" condition occurs which indicates that the incoming signal is in a different band than that for which the scaler and counters are preset. Thus, a first attempt at a count may yield an erroneous result, but the system functions on a time sampling basis to automatically reset the counters and begin the counting cycle again in the next band (with a different set of presets). Regardless of the tuner frequency, the counting system produces a correct indication of the corresponding channel number.
As will be seen, all of the proposed and existing systems suffer from one or more difficulties overcome by the system of the invention. Neither a frequency synthesizer or birdy counter type tuning system is continuous; that is, a "one-time" effort is made to tune to the desired television frequency. In the birdy counter, if the carrier corresponding to the selected channel is not present, or fades after tuning, or if for any reason the birdy count is lost, the system tuning becomes indeterminate and tuning must be reinitiated; that is, the birdy count must begin again. Elaborate filtering is also required to eliminate the harmonics which may cause picture or tuning interference, or both. As mentioned in one of the above papers, an improved birdy counting system employs means for "slowing down the count" to prevent overshooting the desired carrier frequency as the receiver tuning frequency approaches the desired signal frequency. However, the other deficiencies of birdy counting systems are still present. The frequency synthesizer operates on an error signal and should the frequency deviation generate an error signal outside the locking range of the phase lock loop, the system loses tuning.
The Dexter digital automatic frequency control (DAFC) system alleviates the problem of tuning dial inaccuracy and local oscillator drift. Dexter monitors the least significant digit of the oscillator frequency digit. However, he describes that certain severe environmental conditions, particularly high vibration, could cause oscillator frequency changes in excess of that measurable by the last digit. For example, if the system is set to monitor a last digit of five, an oscillator change of 15, would not be detected. The Dexter solution is to add more digits to the comparator to guard against such gross changes. The Dexter DAFC system thus maintains a single preselected frequency to the degree of accuracy of his counter. The paper discusses narrowing the timing frequency range in conjunction with a closed loop control system to provide an extremely accurate dial readout. It may be recalled that this system is independent of received signals.
Any practical television tuning system must be capable of dealing with signals subject to broadcast frequency discrepancies, signals handled by relay systems, signals affected by transmission problems and those rebroadcast for cable television systems. The last, in particular, may suffer from severe carrier frequency errors. An extremely accurate tuning, independent of the received signal, could cause a severe loss of information, and be unusable. Conversely, a broader acceptable tuning range renders the system susceptible to tuning to spurious signals.
In the direct tuning system discussed, a separate potentiometer is required for each channel and, in an all-channel system, this necessitates 82 potentiometers. Even in the modified form, which accepts channel number input commands encoded into appropriate "addresses" of the corresponding potentiometers, the basic difficulty with any all-channel direct tuning system remains. That is, each channel control voltage must be specified and tailored to the characteristics of the individual varactor tuner.
One alternative direct tuning system provides a fixed read-only-memory which is tailored to the individual varactor tuner for supplying the appropriate tuning voltage for each channel. This system encounters the difficulty of the DAFC system in that it either has an insufficient margin for error in system response to accommodate off frequency channels or atmospheric distortions of the broadcast signal, or the acceptable tuning range is so broad as to render it susceptible to operation with spurious or adjacent channel signals. Additionally, it requires programming the memory for all 82 channels though only a relatively few channels may ever be received.
These and other shortcomings in the prior art systems are overcome with the novel tuning system of the invention which is: (1) channel-number-accessed, (2) capable of specifying any designated channel in the discontinuous television bands to within a narrow frequency region (which precludes tuning to adjacent channels or other spurious signals), and (3) capable of accepting signals with minor deviations from their allotted signal frequency. The system is adaptable to any varactor tuner without requiring tailoring for individual characteristics and eliminates costly calibration. The system operation is continuous on a time sampling basis and is not disturbed by variations in the incoming signal, nor will it change tuning, even though the carrier signal disappears.
In accordance with the invention, a digital signal representative of the tuner local oscillator frequency is supplied to one input of a comparator with a digital signal representative of a desired channel number being supplied to the other input. The local oscillator frequency and, hence the system tuning, is varied in accordance with a correction signal which is dependent upon the state of the comparator. As a result of the desired channel number input, a driving ramp voltage is applied to the varactor and effectively results in a systematic channel-by-channel search for a predetermined acceptable region of frequencies about the desired channel frequency. The search is carried out at a very high speed, with a repetitive time sampling and comparison technique and, as far as the viewer is concerned, is practically instantaneous. The acceptance region is defined by a "window" which is made narrow enough to define the desired channel without adjacent channel interference, yet broad enough to permit minor deviations in carrier frequency without initiating corrective action.
In the preferred implementation of the invention, the tuning system incorporates a presettable modular scaler which drives presettable units and tens counters. Any base for the modular scaler may be chosen in conjunction with its preset information and the other counters and their preset information) to provide correct decoding of the oscillator frequency into the corresponding channel number regardless of its frequency band location. Thus, having a predetermined modulus (base), the modular scaler preset determines how many counts are required to provide a pulse to the units counter. The units counter functions in a similar manner to supply pulses to the tens counter. The base of the modular scaler is selected to produce a desired relationship for the intrachannel frequency spacing. Within the frequency window maintained by the tuning system, a normal automatic frequency control (AFC) in the receiver operates to lock the local oscillator frequency to the picture IF carrier. Since the tuning system operates on the local oscillator frequency, there is no dependence on individual characteristics of the varactor other than the requirement that the tuner drive system must be capable of tuning the varactor tuner over all channels.